The ability of ultrafast X-ray and electron pulses to probe structural dynamics with atomic spatiotemporal resolution has fueled a wealth of exciting research on the frontiers of physics, chemistry, biology and materials science. Electrons normally have a shorter penetration depth compared to X-rays. However, given the same energy, the scattering cross sections of electrons can be 105-106 times larger than that of X-rays. In addition, high intensity table-top electron sources are relatively more available, thereby favoring the application of electrons in the study of surfaces, gas phase systems and nanostructures.
One technology that employs ultrafast electron pulses to study materials is ultrafast electron diffraction (UED). UED is a form of pump-probe technique that can directly couple to structural dynamics using electron pulses as the probe. In a typical UED setup, an ultrafast laser pulse is split into a first part that is directly focused on to the sample to create a non-equilibrium state and a second part that is frequency tripled and focused on to a photocathode to generate electrons via photoelectric effect. The generated electrons are then accelerated through, for example, a high voltage (e.g., around 30-100 keV) and focused onto the sample. At these energies, the de Broglie wavelength λ (˜0.07 Å) of the electrons is normally smaller than the inter-atomic spacing so that the electrons can be diffracted from the sample and thus employed as a probe. Examining the diffraction pattern of the electron probe as a function of time delay with respect to the first part of the laser pulse can provide both the equilibrium structure and a movie of the structural evolution. In practice, UED can monitor the position, intensity, and width of the lattice Bragg spots as a function of time after the photo-excitation. Furthermore, ultrafast electrons can also be used in a closely related technique called ultrafast electron microscopy (UEM), which may directly record real space images of transient structures with ultrafast time resolution.